Bonkers, I think you should be asking the police for a detailed explanation as to why the case was NFA'd and what investigative enquiries were made. You need to know who made the decision, was it the police or CPS.
If the police made the decision ask why it wasn't referred to the CPS and that you want the case reviewed by them. If it was the CPS then ask for them to make contact with you to explain their decision. As mentioned above, you can ask the CPS to review the case.
The police may be able to satisfy you that the decision was the correct decision after being given a detailed explanation, if you disagree then ask to see the duty inspector or station commander and request they review it along with the CPS.
If that fails then IPCC i guess.